New material shines light on magnons

All magnets – from the simple trinkets hanging on your refrigerator to the hard discs that store data in computers to the powerful versions used in research labs – contain spinning quasiparticles called magnons. The direction that one magnon spins can influence that of its neighbor, which affects the spin of its neighbor, and so on, yielding what are known as spin waves. Information can potentially be transmitted via spin waves more efficiently than by electricity, and magnons could serve as ‘quantum interconnects’ that ‘glue’ quantum bits together into powerful computers.

Magnons have enormous potential, but they are often difficult to detect without bulky pieces of lab equipment. According to Xiaoyang Zhu, a researcher at Columbia University, such setups are fine for conducting experiments, but not for developing practical magnonic devices and so-called spintronics. The process of detecting magnons can be made much simpler, however, with the right material.

Such a material – a magnetic semiconductor called chromium sulfide bromide (CrSBr) that can be peeled into atom-thin, 2D layers – has now been synthesized in the laboratory of Xavier Roy, a professor in the Department of Chemistry at Columbia University. In a paper in Nature, Zhu, Roy and collaborators at Columbia, the University of Washington, New York University and Oak Ridge National Laboratory report that the magnons in CrSBr can pair up with another quasiparticle called an exciton, which emits light, offering a way to ‘see’ the spinning quasiparticle.

As the researchers perturbed the magnons with light, they observed oscillations from the excitons in the near-infrared range, which is nearly visible to the naked eye. “For the first time, we can see magnons with a simple optical effect,” Zhu said.

According to first author Youn Jun (Eunice) Bae, a postdoc in Zhu’s lab, this effect may be viewed as a quantum transduction, or the conversion of one ‘quanta’ of energy to another. The energy of excitons is four orders of magnitude larger than that of magnons, but because they pair together so strongly, the researchers could easily observe tiny changes in the magnons, Bae explained.

This transduction may one day allow researchers to build quantum information networks that can take information from spin-based quantum bits – which generally need to be located within millimeters of each other – and convert it to light, a form of energy that can transfer information up to hundreds of miles via optical fibers.

The coherence time – how long the oscillations can last – was also remarkable, Zhu said, extending for much longer than the five-nanosecond limit of the experiment. The effect could also travel over 7µm and persist even when the CrSBr devices were made of just two atom-thin layers, raising the possibility of building nanoscale spintronic devices. These devices could one day provide more efficient alternatives to today's electronics. Unlike electrons in an electrical current, which encounter resistance as they travel, no particles are actually moving in a spin wave.

This work was supported by Columbia’s Materials Research Science and Engineering Center (MRSEC), with the material created in the Energy Frontier Research Center (EFRC). The researchers now plan to explore CrSBr’s quantum information potential, as well as other material candidates.

“In the MRSEC and EFRC, we are exploring the quantum properties of several 2D materials that you can stack like papers to create all kinds of new physical phenomena,” Zhu said. For example, if magnon-exciton coupling can be found in other kinds of magnetic semiconductors with slightly different properties than CrSBr, they might emit light in a wider range of colors. “We’re assembling the toolbox to construct new devices with customizable properties.”

This story is adapted from material from Columbia University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.